OPK discrepancy

Has anyone ever gotten 2 different results on 2 different OPKs? On CD 13, I got a solid smiley (peak fertility) on the Clearblue Advanced opk, but negative readings on the cheap (Answer brand) strips on CD 13 and 14. On the afternoon of CD 14, I had some pretty noticeable pain in my ovary for a couple of hours, so I thought I ovulated then. Then, this morning (CD 15), finally got a positive on the Answer brand opk strip! My temperature also rose a little above my coverline this morning. I guess I should see if my bbt stays up to confirm if/when I actually ovulated. In the meantime, I'm just wondering if anyone has any advice about which opk I should consider the most accurate?